There was a sheet finisher which perform a sheet finishing such as punching or saddle-stitching of sheets printed by an image forming system such as a copier, a printer, and a multi-functional peripheral (MFP).
Functions of the sheet finisher become diverse. There is suggested a sheet finisher which has a function of folding a part of sheets or a function of saddle-stitching which saddles an intermediate part of the sheets after the intermediate part of the sheets is stapled in addition to the functions of punching and saddle-stitching (see JP-A-2004-59304, JP-A-2003-182928, etc.).
The sheet finisher that has the saddle-stitching functions is also capable of producing (binding) plural printed sheets into a book.
In the known intermediate saddle-stitching functions suggested in the past, the intermediate part of sheets is stapled and the stapled part of the sheets are saddled by a pair of rollers called a pair of saddle rollers. In this case, a plate-shaped member called a folding blade is put on the stapled portion of a bundle of sheets and inserts the bundle of sheets into a nip of the pair of saddle rollers to form the folding line on the bundle of sheets.
There is disclosed a technique for discharging the sheets by force, if it is detected that a series of discharge operations are canceled due to jam occurrence or the like during stacking sheets (for example, see JP-A-2000-177919, etc.).
However, if it is difficult to discharge sheets forcibly in jam occurrence in the vicinity of a sheet discharging mechanism of the sheet finisher, the sheets stopped in an erect tray have to be removed. In this case, it is difficult for a user to remove the sheets since an opening is narrow in a state where jogger fences of the erect tray guide the sheets.
If the sheets are relatively small in size, it is more difficult for the user to reach and remove the sheet. That is because the sheet finisher stacks the sheets on the erect tray at a location which is away from the opening.
In the past, there was a method of detecting the upper portion of the books which a saddle tray stacks by using a sensor in order to detect a fullness state of the saddle tray and the like subjected to folding. Moreover, there was suggested a method of calculating and estimating the fullness state using the number of sheets and the size and weight of the sheets discharged to the saddle tray. However, it is determined that the saddle tray is full even in a case of the half of actual stacking limit, since it is necessary to operate the sheet finisher in consideration of a large margin in the actual fullness state.
If a job for detecting that the saddle tray is full is canceled like the above method and stacking the books up to the mechanical limit of the saddle tray is intended, resistance of the stacked books causes a torque limiter or the like to operate and thus it is difficult to push a new book, thereby resulting in a discharge error.